With a broadband connexion, IP addresses are attributed dynamically by your Internet Service Provider (ISP)…, for only a limited time which depends on the ISP policy.

In Belgium, this is actually the case. The IP address is renewed periodically by the ISP or by the customer if he switches off/on his broadband modem.

Hence, an IP blocked might be useful for immediate actions like fighting spam. However, with the change of IP address, the hacker or spammer might come back with a new IP address.
Moreover, you might prevent the access of regular user, who is unfortunately customer of the same ISP in the case this regular user get the previously banned IP address.